On a Cross-Diffusion System with Independent Drifts and no Self-Diffusion: The Existence of Totally Mixed Solutions

Abstract: We establish the global existence of weak solutions for a two-species cross-diffusion system, set on the 1-dimensional flat torus, in which the evolution of each species is governed by two mechanisms. The first of these is a diffusion which acts only on the sum of the species with a logarithmic pressure law, and the second of these is a drift term, which can differ between the two species. Our main results hold under a total mixing assumption on the initial data. This assumption, which allows the presence of vacuum, requires specific regularity properties for the ratio of the initial densities of the two species. Moreover, these regularity properties are shown to be propagated over time. In proving the main existence result, we also establish the spatial BV regularity of solutions. In addition, our main results naturally extend to similar systems involving reaction terms.